The present invention relates to nondestructive inspection, and in particular, to methods of pulse thermographic nondestructive evaluation of subsurface defects.
Thermographic nondestructive evaluation (TNDE) is a nondestructive evaluation technique that may be capable of evaluating large areas of structures in a relatively short duration of time to detect and/or quantify sub surface defects. TNDE may be used, for example, in aerospace and other applications to detect and measure the extent of damage to structural materials including, for example, carbon epoxy composites. TNDE techniques may include active and passive thermographic techniques. Active thermography includes an external source of heat that may be applied to an object while passive thermography may use heat generated within the object. Pulse thermography, an active thermographic technique, includes a pulse of heat energy that may be applied to the surface of the test object, usually by a flash lamp. The heat energy may cause a rise in the temperature of the surface of the test object. Following this temperature rise, the rate of change of temperature across the surface as a function of time may be monitored using an infrared camera. In a defect free sample, the heat may diffuse through a thickness of the object, resulting in an asymptotic drop in temperature across the surface. However, in areas where there are defects, the diffusion of heat through the thickness may be obstructed, leaving the surface in areas including defects warmer than in defect free areas. The variation of temperature versus time may be used to evaluate defects.
Some TNDE techniques may require quantification of multiple unknowns, including thermal diffusivity of the material, lateral dimension and depth of defects, thickness of the part being inspected, and/or the amount of heat absorbed by the part from the flash. In some techniques, reliable calibration specimens may facilitate the characterization of the defects. The existence of defects may be qualitatively seen in raw thermographic images. Quantitative information can be obtained with additional image processing techniques, including pixel based and image based techniques. Pixel based techniques may evaluate the temperature evolution of a single pixel or point on the surface. Image based techniques may evaluate the thermal contrast of the entire image per time frame.
Some TNDE techniques may include one or more difficulties in evaluating defects. For example, a thermal camera may introduce noise that may be of the same order as temperature differences to be measured, particularly if a defect is located deep inside the material. A flash heating may not heat the surface uniformly. It may be difficult to detect defect free locations to compare to the locations including defects.
In some TNDE techniques, it may be difficult to generate numerical simulations that compare with the experimental results. For example, the thermo-mechanical properties of the subject material may not be available. Moreover differences in the amounts of heat absorbed, the ambient temperature, the thickness of the sample, etc. may yield differences between the experimental results and the numerical simulations that may make it difficult to evaluate defects.